Can I extrude a plane into a half cylinder?
Or you can use proportional editing.

Get into front view (View &gt; Viewpoint &gt; Front)
Move the middle vertices by your 'radius' (that'd be 100 mm in my example)



Check ✓ Proportional editing
Set Falloff &gt; Sphere
Set Size to 'your radius' (0,1 corresponds to 100 mm)
Check ✓ Projected 2D